1) Field of the Invention
The present invention relates to a crystallized glass having natural-marble-like surface patterns, more particularly to a crystallized glass of a pyroxene type, in particular, a crystallized glass of a diopside type, for use as a building material, and a method of producing the crystallized glass.
2) Description of the Prior Art
Crystallized glasses obtained by subjecting glass to a heat treatment have superior characteristics which make them very desirable as building materials in place of natural stones such as marble or granite. Conventionally, .beta.-wollastonite or forsterite types of crystallized glasses have been mainly utilized in crystallized glasses used as building materials, but, recently, crystallized diopside (CaO.MgO.2SiO.sub.2) and enstatite (MgO.SiO.sub.2) which have particularly superior resistance to chemicals and superior mechanical strength have been attracting a great deal of attention for use in such applications. For example, methods of producing pyroxene type crystallized glass using TiO.sub.2 or fluorine as a nucleating agent have been disclosed in Japanese Laid-Open Patent Application 61-256940 and in Japanese Laid-Open Patent Application 62-108742.
I the methods of producing pyroxene type crystallized glass disclosed in Japanese Laid-Open Patent Application 61-256940 and in Japanese Laid-Open Patent Application 62-108742, a fused glass is first molded into a predetermined shape to make a base glass by a roll-out method, then the base glass is crystallized by a heat treatment. For this recrystallization, a nucleating agent is an indispensable component. However, TiO.sub.2 used as a nucleating agent is so expensive that it is difficult to hold down the raw material costs. Fluorine, on the other hand, is a noxious material and special equipment is required to prevent its escape into the atmosphere. In addition to these problems, with the above-mentioned method it is ver difficult to control the various process conditions such as the molding temperature when the base glass is made. This has the effect of restricting the production, and because the base glass remains under strain, an annealling furnace is necessary, making it difficult to produce this crystallized glass in large quantity at low cost.
In addition, it is desirable that the heat treatment temperature, when the crystallized glass is reheated after crystallization to carry out a bending operation, be 800.degree. C. or lower. Specifically, at this temperature, the dies and other incidental equipment used in the bending process are damaged, and because the formed crystals are melted at the high temperatures, the inherent characteristics of the crystallized glass deteriorate. In addition, because large quantities of pyroxene type crystals must be contained to obtain the desired characteristics with the above method, the crystallinity is inevitably high. The result is that the deformation temperature (Tf) of the crystallized glass obtained is high and it is difficult to perform the bending process at a heat treatment temperature below 800.degree. C.
Another problem inherent in this method is that, even by slight changes in the amount of a nucleation agent contained and in the crystallizing conditions, it occurs that the crystals do not grow uniformly, so accordingly, it is difficult to obtain the desired natural stone patterns.